a. Galaxies are a collection of billions upon billions of stars, planets, dust, and gases, all bound together by gravity and traveling through the universe together. They all are orbiting around the center of the galaxy and traveling as a group.
Stars are not scattered evenly in space, they are grouped, clumped, into galaxies.
When you look up into the night sky, most of the points of light you see are really galaxies, far, far away. When you look at the night map of the USA, each city appears to be a spot of light. The individual street light, each house light, is too small and dim to be seen. But, group all the street and house lights together and you can see a point of light where a city is located.
This is the same effect of galaxies. The individual stars are too small and dim, but put billions of them together and we see a point of light in the night sky that looks like a star to us here on the earth. If we look at a section of space through a telescope we can see the many points of light we think are stars are actually galaxies.
Galaxies are classified by their size and shape, also called the “type” or “kind”.
b. Size, or mass based on number of stars
- Dwarf galaxies have 100 Million stars
- Large galaxies have 100 Billion stars
- Giant galaxies have 100 Trillion stars
3 Shapes – types, or kinds, of galaxies: SPIRAL ELLIPTICAL and IRREGULAR
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II. Spiral galaxies
c. Spiral galaxieds are the most common type of galaxy. They have a central disk, or core area, of bright stars and flattened arms that extended outward and spiral around the nucleus, making the spiral shape.
NGC 1232 in the southern constellation of Eridanus. Overhead View (Bird’s eye view)
The center is brighter because there are so many stars, packed in so tightly. Spiral galaxies can have just two arms or several. The arms can be loose or tightly wound. The center disk can be spherical, however, most have an elongated central region called a bar, these are referred to as a barred spiral galaxy.
d. Barred galaxies have a straight bar of stars that run through the center region.
e. The arms consist of billions of young stars, gas, and dust.
Our Milky Way galaxy is a spiral, and our solar system is located on one of the arms.
The core of a spiral galaxy is called the “galactic bulge”, it is a dense bright region.
This is a side view of NGC-4565 showing the central bulge of a spiral galaxy.
Many spirals may also contain supermassive black holes in their cores.
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III. Elliptical galaxies The most common shape galaxy.
a. Elliptical galaxies vary in shape from nearly spherical(soccer ball) to very elongated (rugby ball).
b. Elliptical galaxies have mostly older stars, and contains very little dust & gas.
c. They are usually dimmer galaxies and are brightest in center region.
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IV. Irregular Galaxies
a. Irregular galaxies have no particular shape, they can be a blob or spread thin.
b. Irregular galaxies have a low total masses, they are not large galaxies.
c. They are rich in dust and gas.
d. Irregular galaxies make up a small percentage of total number of observed galaxies
(They are the least common shape)
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V. Milky Way Galaxy
a.b.c. The Milky Way Galaxy is a barred spiral galaxy, its oldest stars are 12 to 13 Billion years old and the youngest stars are located in the spiral arms of the disk.
d. The Milky Way is part of the “Local Group” of galaxies. 
e. At night, if you are a dark sky area, you can see a “cloudy” path of stars in the sky stretching from one side to the others.
f. When we look through the edge of our galaxy the billions of stars create a path (a.k.a. a “way”) with a milky glow, hence the name “Milky Way”.
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VI. Clusters and Groups – Galaxies which are loosely bonded together by gravity.
a. b. The Milky Way is part of a cluster called the Local Group which consists of more than 54 galaxies including.
The Andromeda Galaxy is our closest major neighbor, it is a spiral galaxy and can be seen with the naked eye in a dark sky area. It looks like a large “smudge” or hazy patch about the size of the full moon. It is easiest to see in the autumn sky, in the east at sunset to overhead about midnight and high in the west at sunrise.
Look for Andromeda galaxy in the early evening, in the eastern sky. The easiest way to find it is by finding the “W” of Cassiopeia and then use the right hand “V” to point to it. Triangulum will be below it but is harder to see.
Andromeda Galaxy Triangulum Galaxy
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VII. Quasars – are very bright, luminous objects. Newest evidence suggests that quasars are produced at the accretion disk around a massive black hole. As the matter is pulled into a black hole’s gravity well it heats up to millions of degrees and is given off as energy. The black hole’s (remember it really is a star’s collapsed core) magnetic field blasts the energy into space at the magnetic poles. BUT… scientists are still not sure.
Quasars are located in the center of many galaxies.
Artist’s creation of the energy being given off at the magnetic poles of a black hole.
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Galaxy Vocabulary
- Black Hole – the collapsed core of a very massive star, light cannot escape it
- Edwin Hubble – he discovered that galaxies are moving away from each other
- Elliptical – a spherical or flatten shaped galaxy, a faint galaxy with older stars
- Galaxy – a collection of billions of stars, gas, and dust all bound by gravity
- Irregular – a galaxy with no regular shape, the least common shape
- Light Year – a measure of distance, how far light travels in one year
- Local Group – 54 galaxies all bound by gravity to each other, the Milky Way is a part of the Local Group
- Milky Way – the name of our galaxy, a barred spiral galaxy
- Solar System – planets and everything that orbits a star
- Spiral – a galaxy with arms that pinwheel around the center, the most common shape galaxy
- Quasar – an enormous amount of energy being given off from the disk of a black hole
My Earth Science 